I really want to get a thunderus but i cant.

That's because it was 1st day Thundurus was released. On 1st day it's always hard to get into raids because demand is so high - everyone wants to get one - while supply is usual, so demand outstrips the supply. It's much easier to get in on 3rd day or during raid hour which happens every Wed 6-7pm local time.

But if you really want to get in the first day here's what you do:
1. Refresh this page every few minutes. Add friend codes only on freshly posted<2min raids. Add asap, you should already have your profile opened ready to type in the codes and send friend requests as you're refreshing that page.
2. After adding 1-3 codes, put your phone right in front of you and watch the lower right hand corner (your pokemon radar) to highlight in orange for a raid invite. As soon as it does, click on it and go to Raids and accept the invite. Keep watching your phone for 2-3 min. If no raid invitation incoming, switch back to that reddit page and keep adding codes.

Mega raids are much easier to get into, since so few people are interested in them. So you might get invited so a mega raid simply because it didn't happen - the host could gather enough players interested in running that raid. Or sometimes you have to wait for invitation longer 5-10 min,raids compared to 1st day legendary where hosts gather enough ppl within 10-20 seconds of posting about their raid.